Clean, high density, soft-accumulating conveyor

ABSTRACT

A conveyor segment for clean manufacturing applications. Each conveyor belt segment includes a pair of side rails parallel to each other; a pair of belt-drives for transporting work pieces from a first end to a second end of the belt segment; a pair of driving wheels for turning the belt-drives. The belt of each of the belt-drives is disposed in a serpentine path about plural, load-bearing upper idler wheels, around either a lower loop-back idler wheel or a driving wheel, and back up and around another pair of upper idler wheels. This sequence is repeated a selected number of times about additional pairs of upper idler wheels and lower loop-back idler wheels or a driving wheel before the belt is disposed beneath plural return idler wheels and back up to the starting point. Slipping between the belt and wheels is thus minimized, even in the absence of a work piece on the belt segment.

CROSS REFERENCE TO RELATED APPLICATIONS

The present utility patent application is a continuation-in-part of U.S.patent application Ser. No. 12/432,129, filed Apr. 29, 2009 and entitled“Clean, High Density, Soft-Accumulating Conveyor,” which claims thebenefit of priority through U.S. Provisional Patent Application No.61/125,901, dated Apr. 29, 2008 and entitled “Clean, High Density,Soft-Accumulating Conveyor”.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

(Not applicable)

BACKGROUND OF THE INVENTION

In some industrial applications of conveyors there are a number ofspecial performance requirements in addition to common parameters suchas speed, weight, and transport capacity. Such applications can be foundin the Semiconductor, Pharmaceutical, Solar Cell, Hard Disk Drive, FlatPanel Display, and other manufacturing industries. For theseapplications and other similar applications, the conveyors used forinter-tool movement of Work In Process (WIP), require “Particulate FreeCleanliness”, “Vibration Free Transport”, “Very High Density WIP Flow”,and “Asynchronous Movements of Pallets with Soft-Accumulation of WIP”(i.e., without collisions or bumping).

Of the above four requirements, current technology has provided forcleanliness, for asynchronous movement, and for soft-accumulation ofWIP, e.g., using precisely guided WIP on rollers, driven by motorscoupled to the wheels via magnetic hysteresis. See, e.g., U.S. Pat. Nos.4,793,262 and 6,047,812. Conventionally, a conveyor transportingmechanism consists of a series of wheels supporting and driving amultiplicity of WIP pallets on each of two parallel sides. A magnetichysteresis coupling allows the driving wheels under a WIP to disengagefrom the drive shaft of the motor if the inertia of the WIP does notpermit synchronization of WIP pallets with the drive speed duringacceleration or deceleration, to avoid the squealing of tires.

Advantageously, magnetic hysteresis coupling reduces rubbing motionbetween driving wheels and WIP pallets, which could otherwise generateparticulates that would adversely impact the clean transportrequirement. Furthermore, magnetic hysteresis coupling, in combinationwith segmentation of the conveyor, provides soft accumulation, i.e.,without bumping, of WIP pallets because the WIP pallets are guided bypresence-of-WIP sensors that define the boundaries of segments on aconveyor that can be occupied by one and only one WIP pallet.

A fundamental drawback of current technology is that the supporting(idling) and driving wheels generate minute vibrations during transportand, therefore, are not able to meet the “vibration-free” requirement.Several physical factors are the cause. First is the near impossibilityof manufacturing a large number of wheels to an absolute same diameterand concentricity. Another factor is the practical impossibility ofdisposing and positioning the wheels to form a straight line, so thatany perfectly-planar WIP pallet riding on it would concurrently touchall of the wheels thereunder.

A further drawback of existing practice is a limitation in achievingvery high density WIP flow due to relatively moderate acceleration anddeceleration rates of the WIP. High density WIP flow requires arelatively close spacing of pallets that travel at high speed. Toachieve this in a collision-free environment and in which pallets maymove asynchronously of each other requires relatively high accelerationand deceleration rates in case one pallet, for whatever reason, slows orstops.

The physical cause of this drawback is the limited surface contactbetween WIP pallet undersides and driving wheels necessary forfrictional adhesion. Indeed, the friction coefficient of soft ordeformable materials is surface area dependent, while hard or more rigidsurfaces is less so. As a result, low settings to initiate earlydisengagement of the magnetic hysteresis drive or clutch, would benecessary, to prevent the spinning of the driving wheels under thepallet during an acceleration mode in which the rubber tires of theclutch-driven wheels are in direct contact with the underside of thedriven pallet.

However, low-torque clutch settings cancel higher acceleration rates ofthe motor driving the clutch. Consequently, high speed and high densityof the pallet flow is not currently achievable. Instead, it is importantto be able to start a pallet from a standing still position quickly andto stop the same pallet traveling in a high-speed transport mode just asquickly, to maintain the high density of flow.

The need for asynchronous movement of the pallets also necessitatesbeing able to transport each pallet individually if there is space tomove the pallet downstream and/or to stop a pallet independently andwithout bumping if another downstream pallet is obstructing its way. Inshort, high speed and high density flow, together, require a firm gripon the pallet during its movements. However, individual driving wheels,with the limited surface contact area with the WIP pallet, currently arenot able to deliver this performance.

To address these shortcomings, existing conveyor segments, which arestructured and arranged to be slightly larger then a WIP or a WIPpallet, can, instead, be equipped with a dedicated drive belt, riding ontop of wheels that are independently driven by the same hysteresisclutch/motor mechanism as before. The high-friction belt, sandwichedbetween the wheels and the WIP pallets, provides necessary adhesionbetween the WIP pallet and the driving, return idler, and/or idlerwheels, to ensure required high, slip-free acceleration. Furthermore,the belt, which is riding on top of the previously disclosed wheels,reduces vibrations generated by any uneven height differences ofsequential wheels.

Disadvantageously, generic belt-driven conveyors are not inherentlyclean. Hence, merely adding belt drives may impact a particulate-freeenvironment. As a result, maintaining a high degree of cleanliness in abelt-driven environment requires special wheel and belt designs.

Accordingly, it would be desirable to provide a high density, highspeed, asynchronous belt-driven conveying system that isparticulate-free, vibration-free, and that employs soft accumulation.

SUMMARY OF THE INVENTION

A first belt-driven conveyor includes a flat, thin belt in combinationwith crowned hysteresis driving wheels and flanged idler wheels. Eachdriving wheel is structured and arranged to drive and center the flat,thin belt while the idler wheels are structured and arranged tolaterally confine the work piece or the object carrying the work pieceusing the flanges on the idler wheels. A magnetic hysteresis clutch orcoupling allows the driving wheels for the belt to disengage from thedrive shaft of the motor whenever the inertia of a work piece or objectcarrying a work piece does not permit synchronization of the work pieceor the object carrying the work piece with the drive speed duringacceleration or deceleration. Indeed, the clutch setting ispre-programmed or keyed so that it does not exceed the friction forcebetween the belt and the work piece or the object carrying the workpiece. When the acceleration exceeds this setting, the work piece or theobject carrying the work piece is decoupled from the motor.

A relatively thin belt thickness is desirable because, although idlerwheels rotate at the same rate, those portions of the idler wheel closerto the axis of rotation, i.e., at or near the root, rotate more slowlyrelative to portions of the idler wheel that are disposed farther fromthe axis of rotation. As a result, any difference in the velocities oftwo surfaces on the flange that contact the work piece or objectcarrying the work piece may result in undesirable rubbing and resultingfrictional particulation. Consequently, relatively thin, relatively flatbelt cross sections are more desirable, to reduce the velocitydifferential between potential points of contact and to maintainrequired cleanliness levels.

In a second system, a relatively thicker belt having a raised edge,i.e., an L-shaped belt, is used to laterally confine the work piece orobject carrying the work piece. In this embodiment, each of the drivingwheels and the return idler wheels are machined to include crowns on theouter peripheral surface on which the belt travels. The center of thecrown radius machined on the driving wheels and on the return idlerwheels, however, is slightly offset relative to the centerline of thebelt by a distance x. This offset centers the L-shaped belts whose beltdimensions, e.g., the cross-section, are not uniform.

A third system is effected by eliminating the flanges of the idlerwheels altogether. More specifically, a third belt-driven conveyorincludes a belt having a rounded or substantially rounded cross-sectionin combination with hysteresis-clutch driving wheels having negativecrowns and idler wheels having negative crowns and guide flanges.

Each of the three embodiments described above divides the conveyor intomodules that include one or more segments. Belt segments are thesmallest element of the whole and are dimensioned to handle and totransport a single work piece or object carrying the work piece at atime. Each belt segment includes a sensor(s) that is/are adapted toconfirm the presence or absence of a discrete work piece or the objectcarrying the work piece within the belt segment. Currently, movement ofan upstream work piece or an object carrying the work piece is realizedonly when one or more sequential downstream belt segments is/arecompletely unoccupied. Hence, forward movement of an upstream work pieceor an object carrying the work piece does not begin until downstreamsegments are totally unoccupied. This, then, defines avelocity-independent minimum distance between work pieces or objectscarrying work pieces.

However, this approach affects work piece density, by delaying theforward movement of an upstream work piece or of the object carrying thework piece until a clear signal is received from a downstream beltsegment sensor. This limitation becomes important once higheraccelerations and decelerations are implemented. In this manner, theaddition of soft belts becomes an enabling technology for higherdensity, higher speed, asynchronous, bump-free flow of work pieces orobjects carrying the work pieces.

A further improvement to current technology is obtained by sensing theprecise location of each of the work pieces or of the objects carryingwork pieces during movement by including more sensors or other feedbackmeans along the path(s) of the moving work pieces or objects carryingwork pieces. Data signals from more sensors increase the granularity ofconveyor segmentation, which then becomes virtually finer than the sizeof the work piece or the object carrying the work piece. At the extreme,if various technologies are applied to the conveyor to locate movingwork pieces or objects carrying the work pieces more precisely, higherwork piece density at higher flow speeds can be achieved, whilemaintaining the asynchronous, bump-free, movement requirements.

Yet another improvement of the presently disclosed arrangement isachieved through the use of a serpentine belt path, thereby urging thebelt into continuous rolling contact with idler and driving wheels andavoiding irregular and intermittent contact between the belt and thedriving and idler wheels, particularly when the belt in the respectiveregion is unloaded.

Also, improvements are obtained in the presently disclosed system whenbelt aligning crowns are disposed, not on driven wheels, but on idlerwheels which may define the serpentine belt path and on idler wheelsdefining the belt return path.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is pointed out with particularity in the appended claims.However, the advantages of the invention described above, together withfurther advantages, may be better understood by referring to thefollowing description taken in conjunction with the accompanyingdrawings. The drawings are not necessarily drawn to scale, and likereference numerals refer to the same parts throughout the differentviews.

FIG. 1 shows a conveyor module having plural belt segments in accordancewith the presently disclosed invention;

FIG. 2 shows a belt segment in accordance with the presently disclosedinvention;

FIG. 3 shows a driving wheel in accordance with the presently disclosedinvention;

FIG. 4 shows a segment of a belt-driven conveyor in accordance with thepresently disclosed invention;

FIG. 5 shows a segment of another belt-driven conveyor in accordancewith the presently disclosed invention;

FIG. 6A shows a segment of yet another belt-driven conveyor inaccordance with the presently disclosed invention;

FIG. 6B shows a detail of a driving wheel for the belt segment shown inFIG. 6A;

FIG. 7 is a perspective view of a portion of a conveyor segmentproviding a serpentine belt path in accordance with an embodiment of thepresently disclosed invention;

FIG. 8 is a cross-sectional view of a portion of the conveyor segment ofFIG. 7; and

FIG. 9 is a schematic side view of plural conveyor segments inaccordance with the embodiment of FIG. 7.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 1 and 2, a belt-driven conveying system (“conveyor”)will be described. The conveyor 10 includes a multiplicity ofinterconnected conveyor modules 15 having at least one belt-drivenconveyor segment 25. The belt segments 25 and conveyor modules 15 can bestructured and arranged in a myriad of patterns to satisfy localtransportation and plant requirements. Each conveyor module 15 isinternally segmented into unit length zones or belt segments 25, whosesize (length and width) is determined by the dimensions of the workpiece or by the object carrying a work piece 19. Indeed, the length of aconveyor module 15 is an integer multiplier of the length of each beltsegment 25 within that module 15.

For example, if the dimension of the work piece or object carrying thework piece 19 is 0.5 meters in length and the conveyor module 15 isapproximately two meters in length, a total of four autonomous,belt-driven conveyor segments 25, which are each slightly larger thanthe 0.5 meter length of the work piece or object carrying the work piece19, would be needed per conveyor module 15. Those of ordinary skill inthe art can appreciate that the size of the work piece or the objectcarrying the work piece 19, the length of the conveyor module 15, andthe length of each belt segment 25 in each module are all variable.

Each belt segment 25 of each conveyor module 15 includes first andsecond side rails 12 and 14. The side rails 12 and 14 are structured andarranged to be mutually in parallel or substantially in parallel. Theside rails 12 and 14 can be elevated to any desired height above aplanar surface, e.g., a floor or slab, and/or suspended from an overheadstructure, e.g., a ceiling or beams.

The first and second side rails 12 and 14 of each belt segment 25 arefixedly coupled, respectively, to first and second sides rails 12 and 14of adjacent belt segments 25 a and 25 b in the same conveyor module 15.Furthermore, first and second side rails 12 and 14 of belt segments 25located at the end of a conveyor module 15 are fixedly coupled,respectively, to first and second sides rails 12 and 14 of end portionsof adjacent conveyor modules 15.

To alter the direction of flow of work pieces or objects carrying workpieces 19 or to branch the conveyor 10 in another direction(s), cornerelements (not shown) are constructed on the basis of the length andwidth of the work piece or the object carrying the work piece 19, toallow free network configuration based on such mathematical modularity.Optionally, vertical lifts (not shown) can be outfitted with discretebelt segments 25 and/or conveyor modules 15, to allow verticalnetworking between conveyors 10 that are disposed at differentelevations.

Each conveyor module 15 includes at least one lateral brace 13, which isstructurally connected between parallel rails 12 and 14, to addstructural support to the belt segment 25 and to the conveyor module 15.Although the lateral braces 13 shown in FIGS. and 2 are disposedorthogonal or substantially orthogonal to each of the side rails 12 and14, struts for lateral bracing, instead, could be crossed, e.g., to forman X (not shown).

Belt segments 25 have modular dimensions that are pre-determinedaccording to the size (length and width) of a work piece and/or of anobject carrying a work piece 19. Moreover, each belt segment 25 isstructured and arranged to provide autonomous transport of a work pieceand/or of an object carrying a work piece 19, to transport the workpiece and/or the object carrying a work piece 19 from one end of thebelt segment 25 to the other end. Accordingly, each belt segment 25includes its own supporting and conveying means and its own drivingmeans and, more specifically, each belt segment 25 includes a pair ofdrive belts and belt-supporting wheels, i.e., idler wheels 18, whichphysically support and convey the work piece and/or the object carryingthe work piece 19, and a motor 11 and a pair of belt-driving wheels 16 aand 16 b that propel the pair of drive belts 20.

Belt Segment

As mentioned above, each belt segment 25 is structured and arranged toprovide autonomous transport of a work piece and/or of an objectcarrying a work piece 19, to transport the work piece and/or the objectcarrying a work piece 19 from one end of the belt segment 25 to theother end. Accordingly, each belt segment 25 includes its own supportingand conveying means as well as its own driving means. The supporting andconveying means provide underlying indirect rolling support to the workpieces and/or to the objects carrying the work piece 19 and transportwork pieces or objects carrying work pieces 19, e.g., pallets, boxes,and the like, from one end of the belt segment 25 to the other. Thedriving means is adapted to provide the inertial force necessary todrive the supporting and conveying means.

Referring to FIGS. 2 and 3, an illustrative driving means is shown. Thedriving means can include a drive motor 11 and first and second beltdriving mechanisms, each of which includes plural driving wheels 16 aand 16 b. The driving wheels 16 a and 16 b are disposed, respectively,on the first and second side rails 12 and 14. An extended drive shaft 17is mechanically coupled to each of the driving wheels 16 a and 16 b.

The motor 11 is adapted to directly drive, i.e., rotate, one of the twodriving wheels 16 a and respective belt driving mechanism and toindirectly rotate the other driving wheel 16 b and belt drivingmechanism via the extended drive shaft 17. The belt-driving mechanismsare synchronized by the connecting drive shaft 17. Consequently,transported work pieces or objects carrying work pieces 19 rest on andare supported by the drive belts 20, which are synchronously driven. Theconnecting drive shaft 17 and its means of attachment to the drivingwheels 16 a and 16 b must also meet design criteria, which excludes thegeneration of contaminating particulates. Accordingly, the designsdescribed below are unique, because they allow the first and secondconveyor rails 12 and 14 to be slightly out of alignment. As a result,the connecting drive shafts 17 may attach to each of the driving wheels16 a and 16 b in a less than a perfectly orthogonal fashion.

Indeed, referring to FIG. 3, without the disclosed connecting driveshaft 17 and driving wheel 16 a configuration, were the connecting driveshaft 17 to enter the driving wheels 16 a and 16 b at a non-orthogonalangle, rotation would induce strain on the shaft 17 and on theattachment flange, forcing one or both to wear excessively. Tocircumvent this problem, the ends 42 of the drive shaft 17 can besubstantially flattened from the round. The flange attached to the wheelhub 31, can be structured and arranged to include a centrally positionedslot 35 to accommodate the flat ends 42 of the shaft 17. The slottedopenings 35 in the flange are counter bored and rounded on the shaftentry side, to accommodate a less than orthogonal shaft 17 withoutstrain. Rotating this assembly will then precess the shaft 17 in theslot 35 freely, eliminating or substantially eliminating any undesiredmaterial wear. Material selection is also important to minimize incidentfriction at the point of insertion of each slot 35.

Preferably, the motor 11 is coupled to a driving wheel 16 and theconnecting drive shaft 17 via a magnetic hysteresis clutch that isintegrated internal to the driving wheel 16. The magnetic hysteresisclutch allows different driving speeds between the drive belt 20 and themotor 11 during acceleration and deceleration. The variable loadassociated with each belt segment 25, e.g., fully-loaded,partially-loaded, and empty, affects the inertia of the work piece orobject carrying the work piece 19.

The hysteresis clutch has an internal, rotary portion and an externalclutch housing, which is the driving wheel 16 itself. The magnetichysteresis clutch is adapted so that the internal, rotary portion isfixedly coupled to, i.e., pressed onto, the rotor or drive shaft of themotor 11 while, due to a magnetic hysteresis effect, the external,clutch housing portion (not shown) is free to rotate asynchronously onthe same rotor or drive shaft. In this manner of operation, whendesired, the motor 11 can continue to drive the internal, rotary part ofthe clutch while, at the same time, the external clutch housing portionis arrested from rotating.

The drive belts 20 are driven by each of the drive wheels 16 a and 16 bthat are mechanically coupled to the external clutch housing portion.Hence, by engaging and disengaging the external clutch housing portion,the motor-clutch combination can be controlled to deliver limiteddriving torque to the belts 20 that is independent of speed. Forexample, if motor torque exceeds the retarding forces on the belts 20and the external clutch housing portion, the clutch housing portion willde-synchronize from the motor drive shaft turning speed. As a result,the external clutch housing portion will rotate at the retarded speed ofthe spinning drive belt 20. Advantageously, while the clutch housing isde-synchronized and rotating at a retarded speed, it continues to exerta pre-established, constant driving torque.

The supporting and conveying means of each belt segment 25 includes apair of rails 12 and 14 that are structured and arranged to structurallysupport the dead load of the driving and conveying means as well as thelive load of a work piece and/or of any object carrying a work piece 19.The work pieces or objects that carry work pieces 19 are in directcontact with and ride directly on the pair of drive belts 20, which,when rotated by corresponding driving wheels 16 a and 16 b, move thework pieces or objects carrying work pieces 19 from one end of the beltsegment 25 to the other end. The drive belts 20 travel along the idlerwheels 18, which are adapted to rotate freely with the belts 20 withoutadding additional driving forces.

Each drive belt 20 is structured and arranged to journey over thefreely-rotatable idler wheels 18. Idler wheels 18 are removably attachedto the first and second side rails 12 and 14, e.g., using bearingcombinations, screws, bolts or rivets having low-friction axles, and thelike, so that the weight of the work pieces or objects carrying workpieces 19 is transferred to the first and second side rails 12 and 14via the drive belt 20 and idler wheels 18. Idler wheels 18 are spacedalong the side rails 12 and 14 at critical intervals, which aredetermined by the belt speed, vibration level, and other designrequirements, as will be discussed further, below.

At one end of each of the first and second side rails 12 and 14 of eachbelt segment 25, opposite the driving wheels 16 a and 16 b, a pair ofidler wheels 18 a and 18 b serves as a return means for the belt 20. Thediameter of the return wheels 18 a and 18 b can be the same orsubstantially the same as the diameter of the driving wheel 16 a or 16 band/or the idler wheels 18 or may be larger or smaller than both.Driving wheels 16 a and 16 b as well as the return idler wheels 18 a and18 b can also be critically shaped to maintain central positioning andtracking of the drive belt 20.

Drive belt lengths are determined by the length of a belt segment 25less the measure or amount of critical stretch of the elastic belt 20for tensioning purposes. Wheel crown cross-sectional geometry fordriving wheels 16 and idler wheels 18 is determined by the beltmaterial, cross-sectional geometry, and the like. Exemplary combinationsof various belts and wheel types will be described below.

Referring to FIG. 4, there is shown a clutch-driven primary drivingwheel 16 b that has a relatively smooth, centering-crown machined ontoits outer periphery. The centering-crown is adapted to center arelatively flat, relatively thin, elastic belt 20. The drive belt 20 isreturned at one end of the belt segment 25 using a similarly-crowned andsimilarly-flanged return idler wheel 18 b.

Between the pair of driving wheels 16 a and 16 b and their correspondingreturn wheels 18 a and 18 b, the drive belt 20 journeys on smaller idlerwheels 18 that include a flange 30. The flanged idler wheels 18 arestructured and arranged to laterally contain the work piece or objectcarrying the work piece 19.

A relatively thin belt thickness is desirable because, although theidler wheels 18 rotate at the same rate, those portions of the idlerwheel 18 closer to the axis of rotation. i.e., at or near the root,rotate more slowly relative to portions of the idler wheel 18 that aredisposed farther from the axis of rotation. As a result, any differencein the velocities of two surfaces on the flange 30 that contact the workpiece or object carrying the work piece 19 may result in undesirablerubbing, which may result in frictional particulation. Consequently,relatively thin, relatively flat drive belt 20 cross sections are moredesirable, to reduce the velocity differential between potential pointsof contact of the work piece or the object carrying the work piece 19and to maintain required cleanliness levels.

Referring to FIG. 5, there is shown a cross-sectional view of anotherembodiment of a belt segment 25 for a system 10 as seen from the returnidler wheel end of the belt segment 25. The motor 11 is mechanicallycoupled to one of the driving wheel 16 a via a magnetic hysteresisclutch. The driving wheels 16 a and 16 b (at the far end of the figure)drive an L-shaped or substantially L-shaped belt 20 that includes araised edged section 33. The long leg of the “L” is disposed on andgenerally in the plane of the peripheral surface of the wheels while theshort leg of the “L” is orthogonal or substantially orthogonal thereto.The confining flanges 33 on the L-shaped belt are structured andarranged to laterally confine the work piece or object carrying the workpiece 19 therebetween.

Between the driving wheels 16 a and 16 b and respective return idlerwheels 18 a and 18 b there are plural idler wheels 18 that, optionally,may include a confining flange 36 (shown in phantom). When a flange 36is included with the idler wheels 18, the bottom, outside corner of theL-shaped belt 20 is guided by the idler wheels 18 at their root.

In this second embodiment, each of the pair of driving wheels 16 a and16 b and the pair of return wheels 18 a and 18 b are machined to includebelt-centering crowns on an outer peripheral surface on which the belt20 travels. Because the cross-section of the L-shaped belt 20 is notuniform, the centers of the crown radius 34 machined on the drivingwheels 16 a and 16 b and on the return idler wheels 18 a and 18 b areslightly offset relative to the centerline 37 of the drive belt 20 by adistance x, to center the drive belt 20 properly. The dimension of theoffset x is determined by the material of the belt, the belt thickness,and so forth.

Referring to FIG. 6A and FIG. 6B, there is shown a belt segment 25having relatively thin, rounded or substantially rounded, elastic drivebelts 20 that are tightly stretched between driving wheels 16 a and 16b, which are disposed at one end of the belt segment 25, andcorresponding return idler wheels 18 a and 18 b, which are disposed atthe other end of the belt segment 25. A first driving wheel 16 a isdirectly propelled by a motor 11 via an internal hysteresis clutch. Asecond driving wheel 16 b is indirectly propelled by a motor 11 coupledthereto by the internal hysteresis clutch and via the drive shaft 17.Between the pair of driving wheels 16 a and 16 b and respective pair ofreturn idler wheels 18 a and 18 b are disposed plural idler wheels 18for guiding the drive belt 20 and for supporting the weight of the workpiece or object carrying the work piece 19.

All of the wheels are machined to include a relatively smooth, reverseor negative crown 38 in their outer peripheries. The negative crown 38is adapted to center and retain the rounded or substantially roundedbelt 20 cleanly, owing to the crown radius 39. Preferably, the radius 39is larger then the radius of the drive belt 20, to minimize cross motionof belt and wheel surfaces, which ensures particulate-free motion.

As an alternative to the use of an L-shaped belt 20, as shown in FIG. 5,or idler wheels 18 a, 18 b having a confining flange 36, as shown inFIG. 4, lateral guide wheels 102 may be provided, as shown in FIG. 7.Such guide wheels are preferably provided with a resilient, somewhatpliable outer surface to avoid jarring a work piece coming into contacttherewith. While effective at confining the work piece to an optimaltravel path, the provision of such lateral guide wheels in all orsubstantially all of the conveyor segments may become costly toimplement. However, the use of lateral guide wheels eliminates the needfor an L-shaped belt or idler wheels with a confining flange. Of course,any combination of an L-shaped belt, idler wheels with a confiningflange, and lateral guide wheels may be provided, though the functionsof each could be regarded as redundant when so combined.

Despite the reduction in particulation achieved through the use of anL-shaped belt or lateral guide wheels, there remains another source ofparticles that has not heretofore been addressed. As is evident in theviews of FIGS. 1, 2, 4, and 6A, each of the depicted belts passesbetween a driving wheel, for example driving wheel 16 a, and a returnwheel, for example, return wheel 18 a. In between, there may be aplurality of idler wheels, such as idler wheels 18, adapted to support awork piece 101 as it passes thereabove. Regardless of the diameter ofthe driving wheel and/or return wheel relative to the idler wheels,there remains the opportunity for the belt to slip across the surface ofone or more of the idler wheels, especially when not loaded by a workpiece. When not actively engaged by the belt, the rate of rotation of anidler wheel may begin to slow. When re-engaged by the belt, the idlerwheel peripheral velocity is less than the linear velocity of the belt.When a work piece passes along the conveyor segment, it forces the beltinto contact with the idler wheel and so accelerates the idler wheel upto the belt speed. Frictional particles may thus be generated due to theinitial difference in velocity.

To prevent such intermittent contact between belt and idler wheels, afurther embodiment of the presently disclosed system and method, asshown in FIGS. 7-9, employs a serpentine belt. Specifically, a drivebelt 103 is disposed about upper idler wheels 104, lower loop-back idlerwheels 105, and driving wheels 108 attached to a respective drive shaft106.

FIG. 7 depicts a portion of one side of a conveyor segment. A work piece101, such as a 300 mm FOUP, is shown on a portion of the segment, and asillustrated would be moved on the opposing belts 103 from the upperright corner of the figure to the lower left corner. Lateral guidewheels 102 are disposed in the side wall on each side of the conveyorsegment. Multiple free-turning upper idler wheels 104 and lowerloop-back wheels 105 are illustrated. A driving wheel 108 attached to adrive shaft 106 is also depicted, as are return idler wheels 107. It isto be understood that a similar configuration of idler wheels anddriving wheel is disposed on the side wall in the foreground of FIG. 7.FIG. 8 is a cross-sectional view through the conveyor segment of FIG. 7taken along lines A-A. FIG. 9 is a side schematic view of multiplesequential conveyor segments, illustrating the belt 103 path of travelthrough the various idler and driving wheels in one embodiment.

Preferably, as shown in FIGS. 7 and 9, the belt 103 passes over theupper side of two upper idler wheels 104, down around the forward sideof the forwardmost of these two upper idler wheels, around the rearward,lower, and forward sides of a lower loop-back wheel 105, then back up tothe rearward and upper sides of the next upper idler wheel. Thus, thebelt passes over two upper idler wheels, down and around a lowerloop-back wheel, then back over the next two upper idler wheels.

In the illustrated embodiment, then, the belt is in contact with over25% of the peripheral surface area of each of the upper idler wheelsmaking up a pair of upper idler wheels. The belt is also in contact withover 50% of the peripheral surface area of the lower loop-back wheelsand of the driving wheel. The belt is also in contact with less than 25%of the peripheral surface area of each of the return idler wheels.

At certain intervals between pairs of upper idler wheels, the beltpasses down and around a driving wheel 108 instead of a lower loop-backwheel. As with the driving wheels of the previously describedembodiments, the driving wheels of FIGS. 7-9 are preferably connectedtogether by a drive shaft 106 in order that the driving wheels on eitherside of a conveyor segment are rotating at the same speed. Here, thedrive shaft is depicted as a simple cylinder, though it is understoodthat the drive shaft can take one of a variety of forms, such as theconnecting drive shaft 17 of FIGS. 3 and 4. Thus, the belt is held intopositive contact with each of the upper idler wheels 104 regardless ofwhether a work piece 101 is riding on that portion of the belt 103.

The driving wheels 108 and drive shaft 106 may be disposed at anysuitable location within a conveyor segment. However, it may bepreferable to provide the driving wheels closer to the center of therespective conveyor segment.

As shown in FIGS. 7 and 9, the upper idler wheels 109 adjacent thedriving wheel 108 have a smaller diameter in order to accommodate thedriving wheel 108. In embodiments where there is a greater amount ofspace between wheels, there need not be this difference in diameter.

At selected positions, the belt passes from the forward side of an upperidler wheel 104 to a return idler wheel 107 that has a lower surfacebelow that of each lower loop-back wheel. Another return idler wheel isdisposed at the opposite end of the respective conveyor segment andguides the belt up into contact with the rearmost upper idler wheel inthat conveyor segment. Optionally, one or more additional return idlerwheels may be disposed along the length of the respective conveyorsegment to guide the belt as it travels in the direction opposite tothat of work piece movement. At least one cross-beam 110 used tomaintain the alignment between the two sides of the conveyor segment isalso shown in FIG. 7.

As is evident from the drawings, the upper idler wheels, lower loop-backwheels and return idler wheels are disposed on the respective side railof the conveyor segment via shafts that extend orthogonal to, orsubstantially orthogonal to, the respective side rail, thereby enablingthe wheels to rotate in a plane substantially parallel to the respectiveside rail. As described, the driving wheels are similarly mounted,though the driving wheels on opposite sides of the conveyor segment areconnected by the drive shaft.

The number of upper idler wheel pairs per conveyor segment may beselected depending upon factors such as segment length, belt material,belt thickness, driving wheel hysteresis clutch characteristics, etc.The length of each belt segment on a linear conveyor is determined bythe dimensions of the work piece to be transmitted. In a typicalpractical application, a conveyor segment transporting 300 mm FOUP is0.5 m long.

In earlier described embodiments, driving and select idler wheels areprovided with centering crowns in order to keep the belt approximatelycentered on each driving and return idler wheel. However, if any of theshafts for wheels having crowns are not orthogonal to the direction ofbelt and work piece travel, the belt may travel in slightly eccentricpaths across the outer periphery of the respective wheel surface, ratherthan along the optimal path otherwise defined by the crown. As a result,the respective wheel may rotate at a speed that may be different fromthe complementary wheel on the opposite side of the conveyor segment.This results in frictional slip under the directly carried work piece,generating some non-negligible degree of particulation. To address thispossible source of contamination in the embodiment of FIGS. 7-9, crownsare preferably disposed only on idler wheels.

Controller

Control of the asynchronous movement and flow of the work piece orobject carrying the work piece 19 can be achieved by embedding amicrocontroller or a network of microcontrollers in the conveyor body.The controller 50 (FIG. 5) includes hardware or software applications toexecute fundamental transport logic, such as asynchronous flow and softaccumulation, i.e., without bumping, linear drive and speed regulation,acceleration and deceleration of the work piece or object carrying thework piece 19, logic controlling the branching into and merging fromplural flow, as well as for tracking the work piece or object carryingthe work piece 19 from a source or point of entry to a destination orexit point. Asynchronous flow on the internally segmented conveyor 10follows the embedded logic where each belt segment 25 is capable ofsensing the presence of a work piece or of an object carrying a workpiece 19 and allows work piece or object carrying the work piece 19entry from a direction of upstream flow if, and only if, that beltsegment 25 is confirmed to be vacant and available, which is to sayempty of any work piece or object carrying the work piece 19. Such logicinherently promotes linear movements and soft accumulation of workpieces or objects carrying the work pieces 19.

An even further improvement on the above control logic includes thepreferential movement of work pieces or objects carrying work pieces 19towards belts segments 25 that are in the process of evacuating arespective work piece or object carrying a work piece 19. The improvedlogic allows higher throughput by increasing flow density on theconveyor 10 and, further, includes time and distance calculation ofphysical positions of discrete work pieces or objects carrying the workpieces 19. Such an improved algorithm can be enhanced by the addition ofadditional sensors 60 (FIG. 2) on each belt segment 25 of the conveyor10, which enables more precise location of discrete work pieces orobjects carrying work pieces 19, than the original segments would allow.

A further improvement towards higher density and higher throughput wouldbe the mechanical reduction of the belt segment 25 size. The reductionwould mean zone segment size less than that of work pieces or objectscarrying the work pieces 19, but still an integral fraction of it.

Many changes in the details, materials, and arrangement of parts andsteps, herein described and illustrated, can be made by those skilled inthe art in light of teachings contained hereinabove. Accordingly, itwill be understood that the following claims are not to be limited tothe embodiments disclosed herein and can include practices other thanthose specifically described, and are to be interpreted as broadly asallowed under the law.

What is claimed is:
 1. A conveyor segment for clean manufacturingapplications, the conveyor segment comprising: a pair of mutuallyparallel side rails; a pair of autonomous belt-drives for transportingat least one work piece from a first end of the belt segment to a secondend of the belt segment, each belt-drive being disposed in parallel orsubstantially in parallel with a respective one of the pair of siderails, each belt-drive comprising plural pairs of upper idler wheelsdisposed at substantially the same height along the respective siderail, a plurality of lower loop-back wheels, each disposed intermediatetwo respective pairs of upper idler wheels and disposed lower, along therespective side rail, than the two respective pairs of upper idlerwheels, a driving wheel disposed intermediate two respective pairs ofupper idler wheels and disposed lower, along the respective side rail,than the two respective pairs of upper idler wheels, a first returnidler wheel disposed proximate the first end of the belt segment and asecond return idler wheel disposed proximate the second end of the beltsegment, the first and second return idler wheels both disposed lower,along the respective side rail, than the plurality of lower loop-backwheels and the driving wheel, and a continuous belt having asubstantially planar cross-section, wherein the belt is disposed incontact with, in sequence: an upper surface of a first pair of upperidler wheels; a lower surface of a respective one of the plurality oflower loop-back wheels or the driving wheel; and an upper surface of thenext, consecutive one of the plural pairs of upper idler wheels, whereinthe belt is disposed in contact with, in sequence: an upper surface ofthe pair of upper idler wheels most proximate the second end of the beltsegment; a lower surface of the second return idler wheel; a lowersurface of the first return idler wheel; and an upper surface of thepair of upper idler wheels most proximate the first end of the beltsegment, wherein the upper idler wheels, lower loop-back wheels, andreturn idler wheels all are adapted to rotate in a plane that issubstantially parallel to the respective side rail.
 2. The conveyorsegment of claim 1, wherein the outer periphery of each of the upperidler wheels, lower loop-back wheels, and return idler wheels has acrown for centering the belt thereon.
 3. The conveyor segment of claim2, wherein the crowns of each of the idler wheels, lower loop-backwheels, and return idler wheels are substantially co-planar.
 4. Theconveyor segment of claim 1, further comprising at least oneintermediate return idler wheel disposed between the first and secondreturn idler wheels and disposed with respect to the respective siderail at substantially the same height as the first and second returnidler wheels.
 5. The conveyor segment of claim 1, wherein eachbelt-drive further comprises plural lateral guide wheels disposed inconjunction with the respective side rail for selective peripherallyrolling contact with a work piece passing through the conveyor segment,the axis of rotation of each of the plural lateral guide wheels beingsubstantially orthogonal to the direction of work piece travel throughthe conveyor segment.
 6. The conveyor segment of claim 1, wherein theouter surface of the driving wheel of each belt-drive is substantiallycylindrical.
 7. The conveyor segment of claim 1, wherein the belt is incontact with over 25% of the peripheral surface area of each of theupper idler wheels making up each pair of upper idler wheels, in contactwith over 50% of the peripheral surface area of the lower loop-backwheels and of the driving wheel, and in contact with less than 25% ofthe peripheral surface area of each of the return idler wheels.
 8. Amethod of conveying a work piece on a conveyor segment, each conveyorbelt segment further comprising a pair of mutually parallel side rails,a pair of autonomous belt-drives for transporting at least one workpiece from a first end of the belt segment to a second end of the beltsegment, each belt-drive being disposed in parallel or substantially inparallel with a respective one of the pair of side rails, eachbelt-drive comprising plural pairs of upper idler wheels disposed atsubstantially the same height along the respective side rail, aplurality of lower loop-back wheels, each disposed intermediate tworespective pairs of upper idler wheels and disposed lower, along therespective side rail, than the two respective pairs of upper idlerwheels, a driving wheel disposed intermediate two respective pairs ofupper idler wheels and disposed lower, along the respective side rail,than the two respective pairs of upper idler wheels, a first returnidler wheel disposed proximate the first end of the belt segment and asecond return idler wheel disposed proximate the second end of the beltsegment, the first and second return idler wheels both disposed lower,along the respective side rail, than the plurality of lower loop-backwheels and the driving wheel, and a continuous belt having asubstantially planar cross-section, the method comprising for eachbelt-drive: disposing the belt in contact with, in sequence: an uppersurface of a first pair of upper idler wheels; a lower surface of arespective one of the plurality of lower loop-back wheels or the drivingwheel; and an upper surface of the next, consecutive one of the pluralpairs of upper idler wheels; and disposing the belt in contact with, insequence: an upper surface of the pair of upper idler wheels mostproximate the second end of the belt segment; a lower surface of thesecond return idler wheel; a lower surface of the first return idlerwheel; and an upper surface of the pair of upper idler wheels mostproximate the first end of the belt segment.
 9. The method of claim 8,further comprising providing a crown on each of the upper idler wheels,lower loop-back wheels, and return idler wheels has a crown forcentering the belt thereon.
 10. The method of claim 8, furthercomprising providing at least one intermediate return idler wheeldisposed between the first and second return idler wheels and disposedwith respect to the respective side rail at substantially the sameheight as the first and second return idler wheels.
 11. The method ofclaim 8, further comprising providing each belt-drive with plurallateral guide wheels disposed in conjunction with the respective siderail for selective peripherally rolling contact with a work piecepassing through the conveyor segment, the axis of rotation of each ofthe plural lateral guide wheels being substantially orthogonal to thedirection of work piece travel through the conveyor segment.
 12. Themethod of claim 8, further comprising disposing the belt: in contactwith over 25% of the peripheral surface area of each of the upper idlerwheels making up each pair of upper idler wheels; in contact with over50% of the peripheral surface area of the lower loop-back wheels and ofthe driving wheel; and in contact with less than 25% of the peripheralsurface area of each of the return idler wheels.